Rotary breather valves for motorcycle engines have been used for many years. The purpose of a breather valve is generally twofold. First, it expels air and oil from the crank case, on the down stroke of the pistons, to the gear case where the oil lubricates various gears and other parts. Second, it draws oil from the bottom of the oil separating pocket (also sometimes referred to as the "settling pocket") in the bottom of the gear case.
Regarding the second purpose, problems arise when the engine is modified to have a larger displacement. The higher horsepower engine results in, among other things, more oil being expelled from the crank case into the gear case (and ultimately the gear case oil separating pocket). If not removed ("scavenged") by the breather valve, this excess oil will leave the engine through the air cleaner system, which is referred to as oil carryover (sometimes also referred to as "blowby" or "puking"). Oil carryover contaminates the air filter and thereby reduces its effectiveness, undesirably enriches the fuel coming into the engine, and pollutes the environment. Excess oil in the gear case can also reduce horsepower.
It can be seen that a new breather valve is needed that improves the scavenging of oil from the oil separating pocket of the gear case.